Long Term Evolution (LTE) systems including LTE and LTE-Advanced, for example, are known as communications systems in mobile communications terminals such as mobile telephones (for example, refer to International Publication No. 2007/091675). In an LTE system, in a connection sequence for initial access to establish UE connection by user equipment ((UE) user terminal) or for connection to a handover destination, a random access channel (RACH) is used by the UE to request an evolutional Node B ((eNB) base station) to assign a communication channel.
Further in an LTE system, a RACH is used for resynchronization and reconnection, and UL/DL resumption (return from a power-saving state). Thus, in an LTE system, for example, compared to a 3G system, the frequency of RACH utilization is several times higher. RACHs include two types, contention based RACHs and non-contention RACHs.
With contention based RACHs, the UE side randomly selects a preamble and therefore, multiple UEs may select the same preamble resource and collisions in the transmitting and receiving of RACH responses and RACH messages 3 have the potential of occurring. Such collisions are responsible for interference in the random access procedure (RA procedure).
A contention based RACH assumes such collisions and, the UE that properly decodes RACH message 3 is ultimately considered the winner UE by the eNB side and the random access procedure is successful. On the UE side, RACH message 3 is used to transmit a contention resolution (resolution) ID that is unique to each UE. If the contention resolution ID returned in RACH message 4 from the eNB coincides with the contention resolution ID of the UE, the UE recognizes the random access procedure to be successful.
On the other hand, with non-contention random access, although the same preamble number is not simultaneously used, 64 preamble resources for one cell are provided, among which a portion is used as dedicated preambles for non-contention random access. Therefore, if the number of UEs in a single cell increases, preamble resources for non-contention random access are depleted causing transition to contention based random access, in turn causing interference in the random access procedure.
In an LTE system in this state, to suppress interference if the UE repeatedly increases the preamble, a back-off indicator that controls the interval at which the UE preamble is transmitted is prepared. Further, if the number of preambles that the eNB receives increases consequent to an increase in the UEs in the cell, the back-off indicator is used to increase the interval at which each UE transmits the preamble, whereby interference is reduced. The back-off indicator provides control on a cellular basis.
In the reception of a preamble from a UE, power ramping is implemented to increase the success rate of the RACH sequence by increasing the preamble output power each time the preamble is retransmitted. However, as described above, with consideration that in the LTE system, the frequency of RACH utilization is high, since the UEs randomly select a preamble, if the number of UEs in the cell increases, it can be assumed that the RACH sequence will be performed simultaneously by numerous UEs. Therefore, the probability of RACH sequence failure consequent to PRACH interference increases.
Further, control that adjusts the preamble transmission interval by the back-off indicator is insufficient in reducing interference when the number of UEs increases. In other words, even if preamble transmission intervals for UEs are made available, if the UEs request the RACH sequence at an identical timing, reduction in interference is not achieved. Further, if the preamble transmission interval is increased by back-off indicator control, the time increases until preamble transmission for resynchronization and UL resumption, thereby inviting increased delays in UE resynchronization and recovery from a power-saving state.
Meanwhile, if the number of UEs in a cell in a LTE system increases, improved system throughput by reductions in interference consequent to the RACH sequence and improved RACH success rates reduce delays in UE resynchronization and recovery from a power-saving state. Therefore, reduction of preamble collisions occurring in the contention based RACH procedure is sought.
Nonetheless, with the conventional contention based RACH schemes above, a problem arises in that since the UEs randomly select a preamble, reductions in interference cannot be facilitated by controlling via the eNB, the preamble that the UEs select.